Recently, in order to supply electric power to an electronic device, a wireless power supply has begun to come into commonplace use. In order to promote the compatibility of products of different manufacturers, a wireless power consortium (WPC) has been organized, and a Qi standard, which is an international standard, has been developed by the WPC.
A wireless power supply that conforms to the Qi standard uses electromagnetic induction between a transmission coil and a reception coil.
FIG. 1 is a view illustrating a configuration of a wireless power supply system 10 that conforms to the Qi standard. The power supply system 10 includes a power transmitter (TX) 20 and a power receiver (RX) 30. The power receiver 30 is mounted on an electronic device such as a mobile phone terminal, a smartphone, an audio player, a game machine, or a tablet terminal.
The power transmitter 20 includes a transmission coil (primary coil) 22, an inverter circuit 24, a controller 26, and a demodulator 28. The inverter circuit 24 includes an H-bridge circuit (full-bridge circuit) or a half-bridge circuit and applies a driving signal S1, specifically, a pulse signal, to the transmission coil 22 such that a driving current flows through the transmission coil 22, thereby allowing the transmission coil 202 to generate an electrical power signal S2 in the form of an electromagnetic field. The controller 26 performs an overall control of the entire power transmitter 20.
In the Qi standard, a communication protocol is defined between the power transmitter 20 and the power receiver 30, which enables information transmission from the power receiver 30 to the power transmitter 20 via a control signal S3. The control signal S3 in the form of an AM (Amplitude Modulation) modulated signal using backscatter modulation is transmitted from the reception coil 32 (a secondary coil) to the transmission coil 22. The control signal S3 includes, for example, electric power control data (also referred to as a packet) for controlling an amount of electric power to be supplied to the power receiver 30, data indicating unique information of the power receiver 30, or the like. The demodulator 28 demodulates the control signal S3 based on a current or a voltage from the transmission coil 22. The controller 26 controls the inverter circuit 24 based on the power control data included in the demodulated control signal S3.
The power receiver 30 includes a reception coil 32, a rectifying circuit 34, a smoothing condenser 36, a modulator 38, a load 40, a controller 42, and a power circuit 44. The reception coil 32 receives a power signal S2 from the transmission coil 22, and transmits a control signal S3 to the transmission coil 22. The rectifying circuit 34 and the smoothing condenser 36 rectifies and smoothes a current S4 induced in the reception coil 32 depending on the power signal S2 to convert the same into a DC voltage VRECT.
The power circuit 44 charges a secondary battery (not shown) using electric power supplied from the power transmitter 20 or steps up or down the DC voltage VRECT to supply the same to the controller 42 and the load 40.
The controller 42 generates electric power control data (also referred to as a control error (CE) packet) for controlling a power supply amount from the power transmitter 20 such that the rectified voltage VRECT approaches its target value. The modulator 38 modulates a coil current of the reception coil 32 depending on the control signal S3 including the electric power control data, thereby transmitting the control signal S3 to the transmission coil 22.
The Qi standard was initially developed for a low power of 5 W or lower of mobile phone terminals, smartphones, tablet terminals, or the like (Volume I Low Power, hereinafter referred to as Low Power standard). Thereafter, the preparation of developing a middle power up to 15 W (Volume II Middle Power, hereinafter referred to as Middle Power standard) is in progress, and the support for a large power of 120 W in the future is planned.
In the Low Power standard, the inverter circuit 24 is configured as a half-bridge inverter, and in the Middle Power standard, the inverter circuit 24 is configured as a full-bridge circuit. When the power transmitter 20 supports both the power receiver 30 of the Low Power standard and the power receiver 30 of the Middle Power standard, the inverter circuit 24 is configured as a full-bridge circuit to change a transmission power within a wide range and a full-bridge operation and a half-bridge operation are required to be switched depending on a transmission power.
The present inventors reviewed switching between the full-bridge operation and the half-bridge operation and recognized that such switching caused discontinuity of transmission power from the power transmitter 20, further, discontinuity of rectified voltage VRECT in the power receiver 30. The discontinuity causes degradation of the communication quality in the power receiver 30 side and impairs the stability of the power receiver 30. Further, the discontinuity makes an operating point of the power receiver 30 unstable, thus making charging unstable.
In addition, when a rapid increase in the rectified voltage VRECT which results from the operation switching of the inverter circuit 24 is permitted, the rated voltage of the power receiver 30 should be increased and circuit components are required to have high pressure resistance, increasing the cost.
Further, this problem should not be considered as a general recognition of a person skilled in the art but independently recognized by the present inventors.